What resolution are you playing at?
It sounds like you were playing without vsync before, check to make sure thats not being forced on from the driver, and also within the games you are playing.
The most likely culprit though is the 10.6 drivers. I noticed a large degrade in performance with these in TF2 & BFBC2 (not source but still) (along with a few people i talked to with a similar setup). My solution was rolling back to 10.3a, you could go to 10.4, or even 10.5. Performance is stellar now. I'll just wait for 10.7 before upgrading again.